The present invention relates to systems for feeding LPG by injection for internal combustion engines of motor vehicles. Fuel feed systems of this kind are already known for example from European patent EP-B-0 725 205 by the same Applicant and from the corresponding U.S. Pat. No. 5,592,924. Additional patents relating to this field are the Patent EP-B-0 922 851 by the same Applicant and the corresponding U.S. Pat. No. 6,050,237.
According to the prior art, fuel feed systems of this kind comprise:
an LPG tank,
a plurality of injectors associated with the cylinders of the engine, for injecting the LPG in the liquid state into said cylinders,
a manifold or rail for feeding the LPG to said injectors,
a conduit for feeding LPG from the tank to the aforesaid rail,
a return conduit for bringing back to the tank the excess LPG fed to the injectors,
a pump immersed in the LPG inside the tank, to feed the LPG through the LPG feed conduit, and
an electric motor for driving the pump.
In the fuel feed systems of the kind described above, it is necessary-to prevent the electric motor driving the pump to continue operating when the LPG inside the tank falls below a predetermined minimum level. In this condition, the pump starts drawing only vapour and the electric motor is no longer cooled by the LPG present inside the tank, with the consequent risk of failure of the motor. To prevent this condition from occurring, known systems exploit the level sensor usually provided within the tank to interrupt the operation of the electric motor driving the pump and activate an auxiliary petrol feed system with which the engine is also provided.
The drawback of this prior art is that the signal of the level sensor has relatively poor reliability, because it is subjected to inaccurate measurements due to the movements of the vehicle aboard which the engine is mounted. It must also be considered that the motor-pump assembly inside the tank is positioned within a separate container communicating with the remaining capacity of the tank through a restricted passage positioned adjacent to its bottom, whose purpose in fact is to make the part of liquid inside which the motor-pump assembly is immersed less prone to being tossed about as a result of the movements of the vehicle. However, this arrangement entails the fact that the level within said container may be different from the level in the remaining part of the tank, which is measured by the sensor level. Obviously, it would be desirable to control the operation of the motor-pump assembly according to the level within the container within which said assembly is positioned.
To solve the aforesaid problems, the present invention relates to a fuel feed system having all the characteristics set out above and further characterised in that it comprises:
sensor means for measuring the electrical current absorbed by the motor driving the pump, and
actuator means for interrupting the operation of the electric motor driving the pump when the aforesaid sensor means detect a lowering of the current absorbed by the electric motor below a predetermined threshold value.
Preferably, the aforesaid actuator means are also able to emit a signal which activates an auxiliary petrol feed system.
The invention further relates to a method for feeding LPG for an internal combustion engine, using a system having the characteristics which have been indicated at the start of the present invention and characterised in that the electrical current absorbed by the electric motor driving the pump is measured and the operation of said electric motor is interrupted when the measured value of the absorbed current is lower than a predetermined threshold value.
Preferably, in the method according to the invention, when a drop in the current absorbed by the electric motor driving the pump below the aforesaid threshold value is detected, an auxiliary petrol feed system is also activated automatically.
In the preferred embodiment of the system and of the method according to the invention, the aforesaid auxiliary petrol feed system is automatically selected if the engine is started when the temperature of the engine coolant is lower than a predetermined minimum value. Also in the aforesaid preferred embodiment, when the operating temperature exceeds the aforesaid predetermined minimum value, the system automatically switches the fuel feed from petrol to LPG. As described above, the operation with LPG fuel feed continues until, due to the consumption of the LPG in the tank, an absorption of electrical current is detected that is lower than the aforesaid predetermined threshold value, in which case the system automatically switches the fuel feed from LPG to petrol.
Thanks to the characteristics set out above, a much more reliable control is obtained over the correct operation of the electric motor driving the feed pump. The current absorbed by the electric motor when the pump is pumping only vapour drops by 40-50%, on average, relative to the values corresponding to a normal operation of the pump with a suction on a liquid substance. The system is therefore able easily to detect the decrease in the quantity of liquid in the tank below the minimum acceptable value. At the same time, the result of a greater operating reliability is achieved with extremely simple means and at relatively low cost.